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Distinct S wave reflector in the midcrust beneath Nikko‐Shirane volcano in the northeastern Japan arc

1996; American Geophysical Union; Volume: 101; Issue: B2 Linguagem: Inglês

10.1029/95jb02883

2156-2202

Satoshi Matsumoto, Akira Hasegawa,

Geological and Geochemical Analysis

Distinct S waves reflected from a midcrustal seismic velocity discontinuity are detected beneath Nikko‐Shirane volcano in the southernmost part of the northeastern Japan arc. A detailed travel time analysis of the reflected S waves by using data acquired through a dense seismic network temporarily set up in this region shows that this unusual S wave reflector is distributed over an area of 15 × 15 km 2 at depths ranging from 8 to 15 km. The reflector has a conical shape becoming shallow toward the summit of Nikko‐Shirane volcano. Observed amplitude spectral ratios of reflected S waves to direct S waves show that the reflector body has a strong velocity contrast to the surrounding medium and its thickness is of the order of 100 m at most. The reflector body is approximated by two thin layers probably filled with partially molten materials. Cutoff depth for shallow seismicity in this area is 3–5 km above the reflector and becomes shallow toward Nikko‐Shirane volcano, nearly parallel to the reflector. The depth to brittle‐ductile transition zone seems to be prescribed by the existence of the reflector body, which is perhaps a thin magma body.